By ALFRED R. WALLACE, F. R. S.
ERRATIC BLOCKS AND ICE-SHEETS.—(Continued.)
WE must now consider briefly the distribution of erratics in North America, because they present some peculiar features and teach, us much concerning the possibilities of glacier motion.
An immense area of the Northeastern States, extending south to New York, and then westward in an irregular line to Cincinnati and St. Louis, is almost wholly covered with a deposit of drift material, in which rocks of various sizes are imbedded, while other rocks, often of enormous size, lie upon the surface. These blocks have been carefully studied by the American geologists, and they present us with some very interesting facts. Not only are the distances from which they have been transported very great, but in very many cases they are found at a greater elevation than the place from which they must have come. Prof. G. P. Wright found an enormous accumulation of bowlders on a sandstone plateau in Monroe County, Pennsylvania. Many of these bowlders were granite, and must have come either from the Adirondack Mountains two hundred miles to the north, or from the Canadian Highlands still farther away. This accumulation of bowlders was seventy or eighty feet high, and it extended many miles, descending into a deep valley one thousand feet below the plateau in a nearly continuous line forming part of the southern moraine of the great American ice-sheet.
On the Kentucky hills, about twelve miles south of Cincinnati, conglomerate bowlders containing pebbles of red jasper can be traced to a limited outcrop of the same rock in Canada to the north of Lake Huron, more than six hundred miles distant, and similar bowlders have been found at intervals over the whole intervening country. In both these cases the blocks must have passed over intervening valleys and hills, the latter as high or nearly as high as the source from whence the rocks were derived. Even more remarkable are numerous bowlders of Helderberg limestone on the summit of the Blue Ridge in Pennsylvania, which must have been brought from ledges at least five hundred feet lower than the places upon which they now lie. The Blue Ridge itself shows remarkable signs of glacial abrasion, in a well-defined shoulder marking the southern limit of the ice (as indicated also by heaps of drift and erratics), so that Mr. Wright